I ordered new custom wheels (rims) for my 87 m.c. s.s., they don't fit,
the hole in my stock rim is 3", hole in new rim is 2", no go, the supplier swears they should fit, turns out he is a classic mustang guy, I"m trying to find out
spindle size for 65-73 mustang, does anyone know, and is 3" correct for my m.c.?.

 

You will need a 3 in center to fit over the rear axle I just measured mine , in case there is any question your bolt pattern is 5 x 4 3/4 inches. Sorry I didn't check the rotor but the rear end is the same

 

Turns out they won't fit because the hub (spindle) is longer than the
depth of the wheel cavity.